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<h4>Techniques</h4>
<div>
	The preprocessor metaprogramming techniques are presented in example format.
</div>

<h4>Example<u> - Use a local macro to avoid small scale repetition.</u></h4>
<div class="code"><pre>
#define BOOST_PP_DEF(op) /* ..................................... */ \
   template&lt;class T, int n&gt;                                          \
   vec&lt;T, n&gt; operator op ## =(vec&lt;T, n&gt; lhs, const vec&lt;T, n&gt;& rhs) { \
      for (int i = 0; i &lt; n; ++i) {                                  \
         lhs(i) op ## = rhs(i);                                      \
      }                                                              \
   }                                                                 \
   /**/

BOOST_PP_DEF(+)
BOOST_PP_DEF(-)
BOOST_PP_DEF(*)
BOOST_PP_DEF(/)

#undef BOOST_PP_DEF
</pre></div>
<div>
	<b>Tip:</b>&nbsp; It is usually okay to use a standard macro name like <code>BOOST_PP_DEF</code> for this kind of code
	because the macro is both defined and undefined in the immediate site of its use.
</div>
<div>
	<b>Tip:</b>&nbsp; It is easier to verify proper use of the line continuation operator when they are aligned.
</div>
<div>
	<b>Notes:</b>&nbsp; You can extend this example by defining more and different kinds of operators.&nbsp;
	Before doing so, consider using the <i>algebraic categories</i> technique introduced in <a href="../bibliography.html#barton">[Barton]</a>
	or a <i>layered architecture</i> (see for instance <a href="../bibliography.html#czarnecki">[Czarnecki]</a>).&nbsp;
	However, at some point you must type the operator tokens <code>*</code>, <code>/</code>, <code>+</code>, <code>-</code>, etc.,
	because it is impossible to generate them using templates.&nbsp;
	The resulting <i>categorical repetition</i> of tokens can be eliminated by using preprocessor metaprogramming.
</div>

<h4>Example<u> - Use BOOST_PP_EMPTY as an unused parameter in local macro instantiations.</u></h4>
<div class="code"><pre>
#define BOOST_PP_DEF(cv) /* ... */ \
   template&lt;class base&gt;            \
   cv() typename implement_subscript_using_begin_subscript&lt;base&gt;::value_type& \
   implement_subscript_using_begin_subscript&lt;base&gt;::operator[](index_type i) cv() { \
      return base::begin()[i];     \
   }                               \
   /**/

   BOOST_PP_DEF(BOOST_PP_EMPTY)
   BOOST_PP_DEF(BOOST_PP_IDENTITY(const))
</pre></div>
<div>
	<b>How:</b>&nbsp; BOOST_PP_EMPTY() expands to nothing and can be used as an unused parameter.
</div>
<div>
	<b>Note:</b>&nbsp; BOOST_PP_EMPTY with the () never gets expanded.&nbsp;
	The () is necessary to invoke a function-like macro.
</div>
<div>
	<b>Caveat:</b>&nbsp; You cannot safely use concatenation while using BOOST_PP_EMPTY().
</div>
<div>
	<b>Tip:</b>&nbsp; Occasionally, one or two lines are considerably longer than the rest.&nbsp;
	It can often save some work to <i>not</i> align all the line continuation operators without making the code too unreadable.
</div>
<div>
	<b>Tip:</b>&nbsp; Use syntax highlighting on preprocessor metaprogramming macro identifiers such as:
	<ul>
		<li>BOOST_PP_DEF</li>
		<li>BOOST_PP_EMPTY</li>
		<li>BOOST_PP_REPEAT</li>
		<li>...</li>
	</ul>
	It can greatly improve readability.
</div>

<h4>Example<u> - Use BOOST_PP_CAT instead of ## when necessary.</u></h4>
<div class="code"><pre>
#define STATIC_ASSERT(expr) \
   enum { BOOST_PP_CAT(static_check_, __LINE__) = (expr) ? 1 : -1 }; \
   typedef char \
      BOOST_PP_CAT(static_assert_, __LINE__)[BOOST_PP_CAT(static_check_, __LINE__)] \
   /**/

// ...

STATIC_ASSERT(sizeof(int) &lt;= sizeof(long));
</pre></div>
<div>
	<b>Why:</b>&nbsp; Macro expansion proceeds recursively in "layers."&nbsp;
	Token pasting prevents the preprocessor from performing macro expansion,
	therefore it is often necessary to delay token concatenation.
</div>

<h4>Example<u> - Use BOOST_PP_STRINGIZE instead of # whenever necessary.</u></h4>
<div class="code"><pre>
#define NOTE(str) \
   message(__FILE__ "(" BOOST_PP_STRINGIZE(__LINE__) ") : " str) \
   /**/

// ...

#pragma NOTE("TBD!")
</pre></div>
<div>
	<b>Why:</b>&nbsp; Macro expansion proceeds recursively in "layers."&nbsp;
	Stringization prevents the preprocessor from performing macro expansion, therefore it is often necessary to delay stringization.
</div>

<h4>Example<u> - Use BOOST_PP_ENUM_PARAMS (and its variants) or BOOST_PP_REPEAT and BOOST_PP_COMMA_IF to avoid <i>O</i>(<i>n</i>) repetition on lists in general.</u></h4>
<div class="code"><pre>
struct make_type_list_end;

template&lt;
   BOOST_PP_ENUM_PARAMS_WITH_A_DEFAULT(
      MAKE_TYPE_LIST_MAX_LENGTH, 
      class T, 
      make_type_list_end
   )
&gt;
struct make_type_list {
   private:
      enum { end = is_same&lt;T0, make_type_list_end&gt;::value };
   public:
      typedef typename type_if&lt;
         end, type_cons_empty,
         type_cons&lt;
            T0,
            typename type_inner_if&lt;
               end, type_identity&lt;end&gt;,
               make_type_list&lt;
                  BOOST_PP_ENUM_SHIFTED_PARAMS(
                     MAKE_TYPE_LIST_MAX_LENGTH,
                     T
                  )
               &gt;
            &gt;::type
         &gt;
      &gt;::type type;
};
</pre></div>
<div>
	<b>How:</b>&nbsp; BOOST_PP_REPEAT uses simulated recursion (pseudo code):
</div>
<div><pre>
#define BOOST_PP_REPEAT(n, m, p) BOOST_PP_REPEAT ## n(m, p)

#define BOOST_PP_REPEAT0(m, p)
#define BOOST_PP_REPEAT1(m, p) m(0, p)
#define BOOST_PP_REPEAT2(m, p) m(0, p) m(1, p)
#define BOOST_PP_REPEAT3(m, p) BOOST_PP_REPEAT2(m, p) m(2, p)
#define BOOST_PP_REPEAT4(m, p) BOOST_PP_REPEAT3(m, p) m(3, p)
// ...
</pre></div>
<div>
	<i>Note:&nbsp; This is no longer how BOOST_PP_REPEAT is implemented, so the code above is illustrative only!&nbsp;</i>
</div>
<div>
	BOOST_PP_ENUM_PARAMS and its variations use BOOST_PP_REPEAT.&nbsp;
	BOOST_PP_COMMA_IF(I) expands to a comma if I != 0.&nbsp;
	BOOST_PP_INC(I) essentially expands to "I+1," and BOOST_PP_DEC(I) essentially expands to "I-1.".
</div>

<h4>Example<u> - Use a <i>conditional macro definition</i> to enable user configuration of code repetition based on need rather than some "reasonable" upper limit.</u></h4>
<div class="code"><pre>
#ifndef MAKE_TYPE_LIST_MAX_LENGTH
#define MAKE_TYPE_LIST_MAX_LENGTH 8
#endif
</pre></div>
<div>
	Now the user can configure the <code>make_type_list</code> primitive without modifying library code.
</div>

<h4>Example<u> - Use BOOST_PP_REPEAT and a <i>token look-up function</i> to eliminate categorical repetition.</u></h4>
<div class="code"><pre>
// CAVEAT:  My compiler is not standard on arithmetic types.
#define ARITHMETIC_TYPE(I) ARITHMETIC_TYPE ## I

#define ARITHMETIC_TYPE0    bool
#define ARITHMETIC_TYPE1    char
#define ARITHMETIC_TYPE2    signed char
#define ARITHMETIC_TYPE3    unsigned char
#define ARITHMETIC_TYPE4    short
#define ARITHMETIC_TYPE5    unsigned short
#define ARITHMETIC_TYPE6    int
#define ARITHMETIC_TYPE7    unsigned int
#define ARITHMETIC_TYPE8    long
#define ARITHMETIC_TYPE9    unsigned long
#define ARITHMETIC_TYPE10   float
#define ARITHMETIC_TYPE11   double
#define ARITHMETIC_TYPE12   long double

#define ARITHMETIC_TYPE_CNT 13

// ...

#define BOOST_PP_DEF(z, I, _) /* ... */ \
   catch (ARITHMETIC_TYPE(I) t) {       \
      report_typeid(t);                 \
      report_value(t);                  \
   }                                    \
   /**/

BOOST_PP_REPEAT(ARITHMETIC_TYPE_CNT, BOOST_PP_DEF, _)

#undef BOOST_PP_DEF
</pre></div>
<div>
	<b>Note:</b>&nbsp; The repetition of the above example can be eliminated using template metaprogramming <a href="../bibliography.html#czarnecki">[Czarnecki]</a> as well.&nbsp;
	However categorical repetition of operator tokens cannot be completely eliminated by using template metaprogramming.
</div>

<h4>Example<u> - Use BOOST_PP_REPEAT to avoid <i>O</i>(<i>n</i>*<i>n</i>) repetition.</u></h4>
<div class="code"><pre>
#ifndef MAX_VEC_ARG_CNT
#define MAX_VEC_ARG_CNT 8
#endif

// ...

#define ARG_FUN(z, i, _) BOOST_PP_COMMA_IF(i) T a ## i
#define ASSIGN_FUN(z, i, ) (*this)[i] = a ## i;

#define DEF_VEC_CTOR_FUN(z, i, _) /* ... */ \
   vec(BOOST_PP_REPEAT(i, ARG_FUN, _)) { \
      BOOST_PP_REPEAT(i, ASSIGN_FUN, _)  \
   }                                     \
   /**/

BOOST_PP_REPEAT(BOOST_PP_INC(MAX_VEC_ARG_CNT), DEF_VEC_CTOR_FUN, _)

#undef ARG_FUN
#undef ASSIGN_FUN
#undef DEF_VEC_CTOR_FUN

// ...
</pre></div>
<div>
	<b>How:</b>&nbsp; BOOST_PP_REPEAT is implemented is a special way to enable <i>automatic recursion</i>.
</div>

<h4>Example<u> - Use BOOST_PP_IF to implement special case for the first element.</u></h4>
<div class="code"><pre>
#define COMMA_IF(c) \
   BOOST_PP_IF(c, BOOST_PP_COMMA, BOOST_PP_EMPTY)() \
   /**/

BOOST_PP_IF(0, true, false) == false;
BOOST_PP_IF(1, true, false) == true;
</pre></div>
<div>
	BOOST_PP_IF enables convenient generation of lists using BOOST_PP_REPEAT.
</div>
<div>
	<b>Note:</b>&nbsp; <i>THEN</i> and <i>ELSE</i> don't have to be macros.&nbsp;
	However, if at least one of them is a function-like macro, and you want it be expanded conditionally,
	you have to make the other parameter a function-like macro too.&nbsp;
	This can often be done using BOOST_PP_IDENTITY.&nbsp;
	Consider the following example (by Aleksey Gurtovoy):
</div>
<div><pre>
#define NUMBERED_EXPRESSION(i, x) /* ... */ \
   BOOST_PP_IF(                             \
      i,                                    \
      BOOST_PP_IDENTITY(x ## i)             \
      BOOST_PP_EMPTY                        \
   )()                                      \
   /**/
</pre></div>
<div>
	<b>Note:</b>&nbsp; Like in the above implementation of COMMA_IF, the result of BOOST_PP_IF is often invoked and not the <i>THEN</i> and <i>ELSE</i> parameters.&nbsp;
	If the parameters were invoked, the code would not expand correctly, because the BOOST_PP_EMPTY parameter would get expanded
	to nothing before the <b>BOOST_PP_IF</b> would be properly expanded.
</div>
<div>
	<b>How:</b>&nbsp; BOOST_PP_IF is defined for the entire repeat range (psuedo code):
</div>
<div><pre>
#define BOOST_PP_IF(c, THEN, ELSE) BOOST_PP_IF ## c(THEN, ELSE)

#define BOOST_PP_IF0(THEN, ELSE) ELSE
#define BOOST_PP_IF1(THEN, ELSE) THEN
#define BOOST_PP_IF1(THEN, ELSE) THEN
// ...
</pre></div>

<h4>Example:<u>&nbsp; Use arithmetic, logical, and comparison operations when necessary.</u></h4>
<div class="code"><pre>
#define SPECIAL_NUMBERED_LIST(n, i, elem, special) \
   BOOST_PP_ASSERT_MSG(                     \
      BOOST_PP_LESS(i, n),                  \
      bad params for SPECIAL_NUMBERED_LIST! \
   )                                        \
   BOOST_PP_ENUM_PARAMS(i, elem)            \
   BOOST_PP_COMMA_IF(i) special             \
   BOOST_PP_REPEAT(                         \
      BOOST_PP_SUB(BOOST_PP_DEC(n), i),     \
      SPECIAL_NUMBERED_LIST_HELPER,         \
      (elem, i)                             \
   )                                        \
   /**/

#define SPECIAL_NUMBERED_LIST_HELPER(z, i, elem_base) \
   ,                                          \
   BOOST_PP_CAT(                              \
      BOOST_PP_TUPLE_ELEM(2, 0, elem_base),   \
      BOOST_PP_ADD(                           \
         i,                                   \
         BOOST_PP_TUPLE_ELEM(2, 1, elem_base) \
      )                                       \
   )                                          \
   /**/

SPECIAL_NUMBERED_LIST(3, 0, E, S)
SPECIAL_NUMBERED_LIST(3, 1, E, S)
SPECIAL_NUMBERED_LIST(3, 2, E, S)
SPECIAL_NUMBERED_LIST(3, 3, E, S)
</pre></div>

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